As per https://github.com/thunderbird/thunderbird-android/issues/935
Identity selection could be enhanced to make life a lot easier for people who have a catch-all domain, similar to how Thunderbird allows customizing the sender.
Identity selection should be :
-free to edit and modify when writing a message (more or less as the recipient field currently is)
-automatically set with the best value guessed from the recipient values, when replying.
This would be a great feature to differentiate K9 mail from other rigid email clients.